Artist's rendering of what the CSIRS stealth fighter
may
look like. This aircraft would penetrate Soviet (or other) air defence
systems, to carry out reconnaissance or precision strike missions over
heavily defended, high value targets. The aircraft would employ carbon
composite structure, FLIR/TV passive sensors and an advanced passive
detection system, to monitor hostile emissions. Weapons and fuel are
carried internally (Illustration by Mark Kopp).

Editor's Note 2005: This technical primer and analysis
predates all US DoD public disclosures on the F-117A/Have Blue and
B-2A/ATB programs. Therefore some key techniques used by the US
contractors such as planform and edge alignment were not publicly known
at that time. Many of the techniques covered in this article were and
remain used in US low observable designs.

The aerial battlefield of the 1990s is liable to be a very hostile
environment. The USSR is currently completing the develop ment of a
whole new generation of air combat aircraft and AAMs, aircraft designed
to match or exceed the performance of the teen-series fighters,
equipped
with long range look down-shootdown radar and configured to operate in
conjunction with AWACS/AEW class aircraft. The current MiG-25M Foxhound
Foxbat derivative and the new Sukhoi Su-27 Ram-K, apparently a
derivative of the F-111 Tornado class Fencer, both appear to be
equipped with a Soviet development of the Hughes AWG-9 radar/fire
control, a valuable gift from Khomeini's regime.

One may, in fact, reasonably assume that the AWG-9 was copied
down to the last transistor, in the same fashion as the Tupolev and
Shvetsov bureaux duplicated the state-of-the-art B-29, in 1946.

If we set aside the air-superiority role, where the Russians
are preparing the MiG-29 Ram-L, a Mach 2.5 twin-engine, twin-fin air
combat fighter with a reported thrust to weight ratio of 1.4:1, and
focus on penetrating Soviet air-defences, be it battlefield or
homeland,
Western aircraft will have to penetrate not only swarms of
lookdown-shootdown fighters, but also SAM belts, supported by massive
phased array radars, and presumably also AAA point defence systems. As
one may observe, a rather nasty lot to contend with, as a whole.

Up to date, the Western philosophy for penetrating Russia's
air defences has been rather simple - fly very fast, down in the weeds
and use as much ECM as practicable, a concept evidenced by the F-111
and
B-1A, both high performance aircraft with terrain following radar and
comprehensive ECM suites. These aircraft were tasked with long range
strike and strategic nuclear strike missions. Battlefield strike and
inter diction were roles to be covered by fighter bombers, operating in
conjunction with ECM and defence suppression aircraft, very much a
brute force approach to the problem. Unfortunately, all things
eventually come to an end, just as the lack of AEW and lookdown radar
did, in this instance.

Obviously, there will be some deficiencies in the new system,
as the Russians have not had the benefit of incrementally gaining
experience through development, this will provide Western air forces
with some breathing space, however, this period will not last for long
and a solution must be found. In the short term, the B-1 B bomber will
fill the gap; though not such a snappy performer as the B-1A, it has a
radar signature only a fraction of its predecessor's. It will penetrate
at low level to deliver nuclear or conventional bombs or launch the
SRAM
defence suppression missiles and ALCM cruise missiles.

Battlefield strike and interdiction roles will presumably
remain the domain of the F-111 and Tornado, until the Russians
integrate
a high performance lookdown radar with a high performance air-combat
fighter, to match the F-15. In the long term, though, more radical
solutions are necessary. In the tactical role, the USAF is seeking the
Advanced Tactical Fighter (ATF), a twin-engined aircraft with a combat
radius better than 1,000 nm, STOL capability, 2-D exhaust nozzles
(manoeuvre/STOL), conformal or internal weapons carriage, Mach 2 cruise
at 50,000 feet and Mach 1.6 at low level. These aircraft would assume
the strike role of the F-111 and air-air role of the F-15, particularly
for deep penetration missions (local air superiority missions being
covered by lightweight fighters, presumably derivatives of the Rockwell
HiMAT or Grumman X-29A FSW fighter).

Performance above all, these aircraft are liable to have
large
radar and infra-red (IF) signatures; considering though the nature of
the missions to be flown, day/night/all weather, this is hardly a
serious deficiency, as there is little point in minimising one's
electromagnetic signature, when the enemy can simply see you flying in.

In the strategic penetration role, bombing or reconnaisance,
this approach is hardly viable, for obvious reasons. The solution is
the
use of Stealth technology. Stealth technology is a rather general term
applied to the whole spectrum of techniques used to reduce an
aircraft's electromagnetic signature. The aircraft's capability to
reflect radar energy, over a given spectrum, must be reduced, which is
a
very involved task, something the reader will later come to appreciate.
An even more Herculean task is the reduction of the aircraft's IR
signature, as one could hardly imagine a glider in the given role.

On the other hand, the benefits to be gained would easily
offset these difficulties. The target would have no prior warning of a
strike, the bombers could penetrate safely at high altitudes,
efficiently cruising at supersonic speeds. Recce/strike aircraft,
operating in adverse weather or at night, could penetrate battle field
defence systems, aside from fulfilling a strategic role.

Given that the performance of all these systems is adequate,
these aircraft would be nearly impossible to detect, could not be
tracked by fire control systems, and would be vulnerable only to
fighters under visual or close range IR acquisition. Due to the mission
profile possible, i.e. Hi-Hi-Hi, long range missions, e.g. across the
Atlantic (viewing the general attitude of Western Europe toward the
USSR, a very likely necessity in any future major conflict), will be
commonplace. In this sense, it may lead to a redefining of the meaning
of air superiority, rather from the brute force extermination of the
opposition's fighters, in order to gain access to any targets
available,
to a rather more subtle case of being able to hit any target any time,
irrespective of the opposition's fighters. This approach is also more
viable in terms of losses, particularly in view of the Communist Bloc's
numerical superiority.

Some insight into the techniques available for the reduction
of an aircraft's electromagnetic signature may be gained from the
following.

Infra-red Signature

Infra-red radiation is emitted by all heat sources in the
aircraft, whatever they may be. The IR band as a whole comprises all
electromagnetic radiation with wavelengths between 800 um and 1000 um,
though the region of interest here is the near infrared, i.e. shorter
than 10 um. That is because the predominant wavelengths emitted by
bodies at temperatures of the order of hundreds of degrees fall into
this band. The principal source of IR in any powered aircraft is the
propulsion. Jet engines being heat engines with less than 100 per cent
efficiency, they radiate waste energy as heat, in two basic ways. The
tailpipe of the jet engine is a very intense source of IR energy, the
intensity and wavelengths depending very much on the type of engine.
Turbojets have EGTs (exhaust gas temperature) of the order of 1000�C,
though newer turbo fans have turbine EGTs around 1350�C. This leads to
emissions in the 1 to 2.5 um band.

The second major source of IR is the exhaust gas plume,
formed
as the exhaust gases flow out of the tailpipe and expand. The plume, on
dry thrust, is cooler than the tailpipe, particularly with turbofans,
which mix cool bypass air with the turbine exhaust gases. These
emissions cover most of the near IR band. Lighting the afterburner will
dramatically increase the temperature of the plume, it will then
dominate the aircraft's signature. Further IR is emitted by hot parts
of
the engine, particularly the afterburner nozzle.

Aside from these sources, the aircraft is likely to emit IR
from its skin; at higher speeds frictional heating occurs, also
reflected and re-emitted sunlight contributes.

As one can obviously perceive, the IR signature cannot be
eliminated, at the best only reduced. Flying at lower speeds, and using
surface paints which have a similar IR reflectance to the background is
liable to reduce emissions from the aircraft's skin. Emissions from hot
parts of the engine can be screened off by parts of the airframe. The
plume temperature may be reduced by mixing in further cool air, thus
assisting in reducing the temperature of the tailpipe region. The key
to
success in suppressing an aircraft's signature lies in shifting its IR
emissions into regions (wavelengths) which are more heavily attenuated
by the atmosphere (as things are, the near IR is best transmitted in
three 'windows', the 2.5, 4 and 10 micron bands), i.e. allowing them to
fall outside of transmission windows, where they are more readily
absorbed by C02 and water vapours. In this fashion, the heat emitted is
far more difficult to detect, at long distances (a factor of growing
importance, as the Soviets appear to be showing great interest in
passive IR target acquisition systems).

Given that all the above factors are considered and dealt
with, the aircraft's signature may be substantially reduced. (Further
reading, TE March 1982.)

Electromagnetic Emissions

Modern combat aircraft emit electromagnetic waves over a very
wide spectrum. The greatest source is the radar, whether operating in
air-air or air-ground modes, emitting pulses of power up to the order
of
hundreds of kilowatts. Given that an opponent has a warning receiver
equally as sensitive as the radar's own receiver, he will detect the
radar at least a4 twice the distance necessary for the radar to pick up
a return. This means that a radar equipped aircraft does an excellent
job advertising both its position and identity, as a spectral analyser
of one or another sort, coupled with a computer memory file, will
identify the radar quite readily. The obvious solution to this problem
is flying with one's radar set shut down, which, of course, creates
another set of problems, related to the detection of the enemy.

Another source of emissions is the use of radar/radio
altimeters and Doppler navigation systems, all of which rely on the
transmission of beams from the aircraft to the Earth's surface, in
order
to make measurements. The solution would appear to be the use of
inertial navigation and perhaps lasers or millimetric wave systems for
height measurement, as these allow much narrower beams.

Radio transmissions, whether voice or digital datalink, will
also alert an opponent; depending on the type of transmission, they may
also allow the identification of the aircraft.

Electronic countermeasures, particularly jammers, will
likewise indicate an aircraft's location. Though they may succeed in
confusing the radar system to be jammed, other systems, passively
listening, may exploit them to locate the incoming aircraft.

The issue of whether to use or not to use ECM is very complex.

Given that the ECM serves to conceal the aircraft and will
not
reveal its position to listening posts, then its use is appropriate. As
it turns out, the vast majority of current ECM serve rather to confuse
or deceive hostile radar, assuming detection is inevitable.

Aside from all of these, functional, sources of energy, an
aircraft is also likely to radiate lesser quantities of interference,
caused by switching transients of various sorts in the aircraft's
electrical system. This is less of a problem in all-metal aircraft, as
the structure will provide some screening, but may become a problem
with
composite structures, which behave much like lossy dielectrics, rather
than conductors.

As one can observe, success in suppressing the whole spectrum
of emissions hinges on the use of passive sensors, line-of-sight
(laser)
communications, inertial or satellite navigation and the ability to
identify and eliminate any forms of interference generated by onboard
systems.

Radar Signature

The radar signature of an aircraft is a measure of its
detectability by a particular radar system. Electromagnetic waves, as
emitted by radar, for instance, propagate through space only until
confronted with a different medium. Depending on the particular medium,
part of the energy will be reflected back to the source, part will
penetrate into the surface. Given that the medium is conductive (e.g. a
metal aircraft skin) and the waves are at microwave (typically radar)
frequencies, most of the energy will be reflected. However, if the
medium has electrical properties close to that of free space, as far as
the wave is concerned, there will be little if any reflection and the
impinging wave will penetrate, propagating through the material. This
may imply the simple solution of building an aircraft of such material,
however, internal systems, e.g. engine, would then reflect.

Fortunately, these materials can be made lossy (absorbing the
energy and re-emitting it as heat) and then a physical phenomenon known
as the skin-effect occurs; this confines the electrical and magnetic
fields (voltages and currents), generated by the impinging wave, to a
thin surface layer, given by the so called skin depth, characteristic
of
a material at a given frequency.

Given that the skin depth is much smaller than the thickness
of the material, all of the wave which did penetrate will be absorbed.

Radar absorbent materials operate on this principle, incoming
waves cannot distinguish them from free space, but are absorbed and
dissipated as heat once inside the material (one could consider trying
to use a paint with similar properties, however, as it appears, one
ends
up with unreasonable thick nesses - electrical lossy paints can however
assist, as will be seen later).

Another effect which can be exploited to reduce the radar
signature is specifically shaping the aircraft. Assuming the aircraft
is
metal, and therefore reflects most of the energy landing on its
surface, by shaping it a particular way, one can reduce the amount
reflected in the direction the radiation came from. A measure of how
much is reflected is the radar cross section, a hypothetical area which
radiates the same amount of energy as the aircraft, whatever shape it
may be, reflects in that particular direction.

All shapes have particular cross-sections, which vary with
wavelength and direction, however the size of these cross sections can
be radically different. Rounded smooth shapes have low cross-sections,
whereas concave or sharp shapes usually focus energy and reflect a lot.
Corners, or joins between say tail and fuselage, reflect very well.
Engine inlets and tailpipes are exceptionally bad; as it often turns
out, the waves will propagate along them, as in a waveguide (see TE,
June 1982) until they hit either the compressor or turbine blades,
which
reflect them out with added modulation, JEM (Jet Engine Modulation),
easily identifying the aircraft.

Flight decks and radars are also bad, as the canopy perspex
is
transparent to radar, just as the radome is, as both areas are anything
but smooth inside, they have large cross-sections.

Weapon bays or hardpoints carried ordnance are a story on
their own; suffice to say both cause difficulties.

After considering these major factors, one can look at the
array of ECM or communication aerials, some of which will in fact be
tuned to the incoming radar to increase sensitivity (radar warning
receivers - RWR). All of these are likely to re-emit some of the
incoming energy.

One technique to reduce this part of the signature involves
increasing the reflectance of the canopy/radome/cover to incoming
radar;
like this it will appear to be another part of the aircraft's skin,
smoothly shaped, hence possessing a lower signature. This is achieved
by coating canopies with thin (transparent) metallic layers, radomes
are built as multilayered dielectric interference filters (TE, March
1982), transparent to onboard radar, but tuned to be opaque to hostile
radar (typically the B-1B).

These techniques work adequately for some systems; however,
wide band ECM aerials need to operate at the same wavelength as the
hostile radars encountered, therefore, they cannot be concealed this
way.

Likewise, inlets and exhausts need to be open, for obvious
reasons. These cases are dealt by the use of radar absorbent materials
and geometry.

Typically an inlet will contain an S-bend or (appropriate)
baffles, concealing the face of the compressor from direct radiation.
As
the walls and baffles are absorbent, the travelling wave will disappear
long before it could reach the compressor.

In summary, one must deal with the cumulative effects of a
large number of smaller component cross-sections and the radar
cross-section of the airframe. The biggest problem would be the design
of a system to counter radars operating at long wavelengths (metres)
down to very short wavelengths (centimetre, millimetre band), as the
cross-section varies to a great degree, with extremes of frequency.

A smooth, flattish shape with a low head-on/side-on physical
cross-section should perform well for most wavelengths, head on or
side-on, but may reflect a lot, if directly above a shorter wavelength
radar. Therefore it would be desirable to employ lossy paints and
absorbent materials all over the surface, to reduce further any
reflection which occurs. Here is where the snag appears, as lossy
paints
and absorbers are usually lossy and absorbent, to the right degree,
only at some wavelengths, and tend to deviate with wavelength.

In that sense, one could build an excellent Stealth aircraft
for one particular family of radars, but it wouldn't be quite so
excellent for another, in fact it may perform miserably.

Another difficulty stems from the skin depths required, as
long wave length radar has a much greater skin depth than shorter
wavelength systems - i.e. an aircraft invisible to a centimetre band
radar may be easily picked up by a WW 11 300 MHz radar.

The problem, as a whole, tends to become quite complicated.

Composite materials, e.g. carbonfibre reinforced epoxy, may
be
a way out, as they could be doped with specific dielectric and
resistive
additives to bring their electrical properties closer to what is
desired; if the aircraft's skin and airframe are built from such
materials, the overall effective thickness may be increased, allowing
for greater skin depth.

What one could envision as a design approach would be the
choice of shape to provide a minimum cross-section (presumably, all
metal models could then be built and tested to verify this), once this
would be established, structural and skin materials would be chosen for
maximum absorption, over as wide a spectrum as possible. The final
stage
would be the choice of geometry and materials for inlets, exhausts,
canopies and sensor/weapon bays or stations.

The United States' current Stealth programme involves the
development of a strategic nuclear bomber, the Northrop ATB, a
reconnaissance/strike fighter, the Lockheed CSIRS, and an advanced
stealthy cruise missile.

Northrop Advanced Technology
Bomber.

The ATB, or Stealth Bomber, is to become the airborne element
of the US nuclear strike triad, it will replace the B-1B in the
penetration role and carry out long range nuclear strike missions.
Northrop is leading the project, presumably for their great experience
with both ECM and large flying wing aircraft, Boeing and Vought are
co-operating. Total contracts for development are worth $7300 million.
The ATB is a heavily classified project, in fact so classified, that
nobody really knows anything specific, at this stage.

It is assumed the aircraft will be a delta platform flying
wing, as this configuration offers both a low radar cross-section and a
good lift to drag ratio, allowing for efficient high speed cruise.
Initial estimates of the powerplants to be used suggested four high
bypass ratio turbofans, chosen for fuel economy and low IR signature.
Current estimates favour two variable cycle engines (a variable cycle
engine allows for continuous changes of bypass ratio to meet either
thrust or fuel consumption require ments, behaving much like a high
bypass turbofan at one extreme or a turbojet at the other), the
suggested size has also decreased. No specific estimates of crew size
seem to be available, though one could assume two to four men.

Engine inlets and exhausts would presumably lie on the upper
surface of the aircraft, employing inlet S-bends, exhaust baffles and
most likely, fairly long inlet and exhaust ducts.

Airframe and skin structures would be carbonfibre composite.
Weapons would be carried in an internal weapons bay, most likely free
fall nuclear bombs, as the small size would preclude the carriage of
stand-off missiles or cruise missiles.

One could assume a mission profile of the following sort -
takeoff with full internal fuel from the continental US or other safe
airbases, followed by a very steep climb, on full thrust, to a cruising
altitude, likely above 40,000 feet. Once at cruise altitude, the
engines
would switch to a high bypass mode and the aircraft would begin a high
subsonic, or low supersonic cruise to the target area. Longer missions
may require in-flight refuelling. Navigation would employ inertial and
satellite systems, though some form of TERCOM update could be used,
over safe zones. Hostile airspace would be penetrated at medium to
high altitudes, exploiting cloud cover wherever possible to confuse IR
surveillance systems. An ATB would carry a comprehensive passive ECM
system, which could classify and locate all hostile sources of
radiation. This data would be passed on to a graphic image generating
computer, which would synthesise a picture of the landscape, with
lethal zones (volumes of space around SAM/radar/AEW systems) clearly
displayed. The pilot would then steer the aircraft between these zones,
avoiding detection and/or tracking, simply by following his TV screen
or HUD. Targets would be attacked with free fall weapons, though these
may be equipped with inertial or TV (smart image recognising systems)
terminal guidance, which would also allow stand-off ranges of several
miles, useful for nuclear strike.

Active, most likely deceptive ECM would be employed for
penetrating heavily defended zones, this would be employed if hostile
radar were to lock on, at close range, during the terminal strike
manoeuvre.

The ATB is to enter service in 1992, which leaves us a whole
decade for speculation and the US DoD a whole decade to revise their
designs. It is very likely the aircraft and mission profile will
substantially alter, as the USSR refines its air defence structure,
only
time will tell.

Lockheed Covert Survivable
In-weather Reconnaissance/Strike

The Lockheed CSIRS is another advanced project employing
stealthy technology. Scheduled for service entry in the late eighties,
this aircraft is likely to perform a primary role of short to medium
range tactical reconnaissance, reflecting the hopeless case
conventional
fighters, fitted for recce, must contend with in penetrating hostile
air-defence zones at low level. A secondary strike role would tend to
back up this approach, to minimise losses over heavily defended high
value targets.

Lockheed are responsible for the project, probably a natural
choice for their specific experience with high performance penetration
aircraft, as the SR-71 Blackbird. No official releases on details of
the project are available, though some journals have reported details,
Lockheed are tight lipped(". . . we can neither confirm nor deny these
reports. . . ").

Reports indicate the aircraft is the F-25, powered by a
29,000
pound F-101 DFE afterburning turbofan. At this point, we can allow
ourselves more speculation, as other reports indicate a platform much
like the shuttle or some of Lockheed's stealthy drones. With this
amount
of information we could picture the following - a rather compact,
single seat aircraft the size of the F-16E (see illustration), with
composite skins and some structural elements, the remainder titanium or
aluminium. The aircraft would have to be very agile, with high
acceleration, to minimise exposure time over a target. The signature
constraints would force fixed inlet geometry, limiting top speed to
Mach 1.8.

The engine would be buried inside the fuselage, afterburner
nozzle inclusive, bleed or bypass air would create a boundary layer
between the exhaust plume and duct wall. These would be possibly clad
with doped carbon-carbon tiles (space shuttle), to absorb incoming
energy, leaving only a narrow cone, aft of the aircraft, where one
could
directly illuminate the turbine. Due to its shape (it would have a
small, if any, tail) the aircraft would be statically unstable in yaw
and most likely in pitch also.

Pitch/roll control would be provided by trailing edge
elevons,
yaw control by split wingtip speedbrakes (Kevlar/carbon composite). The
aircraft would employ a full authority, redundant digital fly-by-wire
system, possibly derived from an off-the-shelf system as the AN/ASW-44.
Weapons would be carried in an internal bay, as conformal carriage
would be inadequate. A possible configuration could be very much like
the rotating bomb bay of the Buccaneer, but employing interchangeable
weapon/sensor/fuel pallets on the stores half of the bay.

A conformal bomb/ missile pallet has lower drag and signature
than an open bay, when exposed. Rotated into a concealed position, it
would not differ from a closed bay. The choice of weapons or sensors
would depend on missions, one could assume a payload in excess of 1000
lb, for a 500 nm combat radius. Hi-Lo-Hi, all fuel being internal. It
is
unlikely a gun would be carried, but the AMRAAM could possibly be
targeted by passive systems, or used in a captive search mode, carried
in a pallet or bay.

The aircraft would have to carry a comprehensive array of
passive sensors. FLIR would be essential, Low Light TV very useful and
a
low light sensitive derivative of TISEO would be excellent. Rearward
facing sensors could also be employed. A passive detection system, for
any radars or emitters, would be essential, with a 360 degree
capability.

Conformal planar phased array antennae could be employed,
these should be available by 1990.

Data from all these sensors would be processed by a high
speed
image processing and generating computer, feeding real images together
with synthesised symbology and synthesised terrain profiles (computer
memory file in conjunction with inertial nav) onto a wide angle HUD, or
helmet projection visor - the Lantirn HUD could represent a suitable
off-the-shelf system. Head down colour CRTs or other displays would
handle non-critical or status information.

One could assume that after being coated with radar
absorbents, the aircraft would receive a low IR, low contrast grey
camouflage.

Stealth technology is in its beginnings, at this stage it
isn't even apparent whether the concept will prove itself in operation
or become a multimillion dollar flop, though it is fair to assume that
whatever the outcome, a lot will be learnt about the reduction of
signatures and a lot of that will be incorporated in other projects. It
is, in its essence, a massive exercise at seeing without being seen
and it does involve a lot of new technology, which must be integrated
very thoroughly.